1. Field of the Invention
The present invention generally relates to switching regulators and, more particularly, to a step-up switching regulator.
2. Description of the Related Art
FIG. 1 is a circuit diagram of a conventional step-up switching regulator 11.
The conventional switching regulator 11 comprises a drive circuit 12 for generating a rectangular drive signal with a duty ratio which varies depending on a voltage to be output, a switching element Q11 switched in accordance with the rectangular drive signal generated by the drive circuit 12, a coil L11 for shaping an output current of the switching element Q11 and for outputting the shaped current, a reverse current blocking diode D11, and a smoothing capacitor C11.
The drive circuit 12 comprises a control circuit 13 for generating a rectangular drive signal with a duty ratio varying in accordance with a level of a voltage control signal, and a driving circuit 14 for driving the switching element Q11 in accordance with the drive signal generated by the control circuit 13.
FIGS. 2A-2C are waveform charts showing an operation of the conventional switching regulator 11. FIG. 2A is a waveform chart of an output of the control circuit 13; FIG. 2B is a waveform chart of an output of the coil L11; and FIG. 2C illustrates a current loss.
The control circuit 13 generates a rectangular signal as shown in FIG. 2A with a high-level period t11 which varies depending on the voltage control signal supplied to the control circuit 13. The control circuit 13 supplies the rectangular signal to the driving circuit 14.
The driving circuit 14 subjects the rectangular signal supplied by the control circuit 13 as shown in FIG. 2A to voltage-current conversion. The driving circuit 14 supplies the converted signal to the base of the NPN transistor Q11 constituting the switching element. The NPN transistor Q11 is driven by the rectangular drive signal as shown in FIG. 2A so as to control a voltage applied to the coil L11.
As shown in FIG. 2B, the coil L11 outputs an output current I.sub.pk which rises linearly in the high-level period of the rectangular waveform as shown in FIG. 2A and falls linearly in a low-level period thereof. The output current I.sub.pk is output at an output terminal T.sub.OUT via the diode D11. The capacitor C11 is connected to the output terminal T.sub.OUT so that the triangular waveform as shown in FIG. 2B is smoothed and output as a dc output voltage.
As described above, in the conventional step-up switching regulator, the rectangular waveform as shown in FIG. 2A with a duty ratio which varies depending on the voltage to be output is supplied to the base of the switching element for controlling the current flowing through the coil. As shown in FIG. 2B, the output current I.sub.pk of the coil varies such that it rises linearly while the switch is on and falls linearly while the switch is off. Therefore, current portions indicated by hatched areas in FIG. 2C are lost, thus increasing current consumption.